Field-oriented vector control of synchronous motors with additional field winding

This paper presents a new method to control a synchronous motor in such a way to resemble the characteristics of a DC motor. The method suggests including a second field winding to the rotor of a voltage-source-inverter-fed synchronous motor. The angular frequency of the inverter is made equal to the angular rotor speed, (of a self-controlled synchronous motor drive). The added field winding is in space quadrature to the main field winding and is properly excited in such a way as to diminish the direct axis component of the stator current at every load conditions. The motor is controlled to operate with zero power angle from the inverter side and zero direct axis current from the rotor excitation side. Therefore, it operates with minimum stator current and with unity power factor. The addition of the second field winding will not complicate the design because it is just a control winding. This winding may be made with smaller wire cross-section and a larger number of turns. The control on this winding is not complicated and it can be easily created. The synchronous motor along with the added field and the required control loops are simulated and tested extensively. The test results show excellent motor performance in motoring and regenerating modes of operation.     

Proposition of a new method for in-service monitoring of the aging of stator winding insulation in AC motors

A new on-line, nonintrusive monitoring system, able to observe the aging of an ac machine winding insulation is described. The principle of this system consists of detecting small variations of the turn-to-turn capacitances due to the dielectric aging. The authors have performed experiments on accelerated insulation aging as well as on a system able to measure machine winding high-frequency resonance, which depends on turn-to-turn capacitance. First, twisted pairs of magnet wires were submitted to thermal and electrical stresses, in order to establish a connection between the turn-to-turn capacitance with the breakdown voltage and the probability of failure. Then, the changes in the resonance frequency of a simplified stator winding, the "motorette," were measured. Measurements of the currents or of the magnetic field in the vicinity of the machine can be made. The analysis of the experimental data yields information on the turn-to-turn capacitance variations, hence on the insulation aging.     

Detection of shorted-turns in the field winding ofturbine-generator rotors using novelty detectors-development and fieldtest

A method for detecting shorted windings in operational turbine-generators is described. The method is based on the traveling wave method described by El-Sharkawi, et. al. (1971). The method is extended in this paper to operational rotors by the application of a neural network feature extraction and novelty detection. The results of successful laboratory experiments are also reported     

Direct field oriented control scheme for space vector modulated AC/DC/AC converter fed induction motor

This paper investigates a Luenberger flux observer with speed adaptation for a direct field oriented control of an induction motor. An improved method of speed estimation that operates on the principle of speed adaptive flux and current observer has been proposed. An observer is basically an estimator that uses a plant model and a feedback loop with measured stator voltage and current. Simulation results show that the proposed direct field oriented control with the proposed observer provides good performance dynamic characteristics. The induction motor is fed by an indirect power electronics converter. This indirect converter is controlled by a sliding mode technique that enables minimization of harmonics introduced by the line converter, as well as the control of the power factor and DC-link voltage. The robustness of the overall system is studied using simulation for different operating modes and varied parameters.     

AC magnetic field enhancement oxygen evolution reaction of bimetallic metal-organic framework

Cost-effective oxygen evolution reaction (OER) electrocatalysts play a key role in electrocatalytic water splitting process. Here, a facile and scalable strategy was applied to synthesize the bimetallic metal-organic frameworks (MOFs) with high OER activity, and the effects of AC magnetic field on OER was also investigated. Results shows that the bimetallic MOFs (Co_0.4Ni_0.6-MOF-74) exhibited a three-dimensional flower-like morphology, and possessed a higher BET specific area of 905.39 m² g^?1 as well as a smaller median pore size of 0.49 nm as compared to single metal MOFs; It owned a lowest overpotential of 314 mV at 10 mA cm^?2 and Tafel slope of 79.39 mV dec^?1, both are much lower than these of single metal MOFs, being due to the high specific area and more active sites derived from the distorted crystal structure; When AC magnetic field strength equaled to 5.50 mT, overpotential at 10 mA cm^?2 for Co_0.4Ni_0.6-MOF-74 reached minimum value of 201 mV, reduced by about 36% as compared to that without magnetic field, indicated that AC magnetic field could greatly improve OER process. These improvements resulted from the spin polarization effect, magnetohydrodynamic (MHD) convection and improved active point temperature.     

Multiphysics DC and AC models of a PEMFC for the detection of degraded cell parameters

The present paper proposes a new 2D modelling of ac impedance spectra of polymer electrolyte fuel cells (PEMFC). The computational domain includes the Membrane Electrode Assembly, the Gas Diffusion Layers and the channels on both the anode and cathode sides. The model takes into account the main fuel cell phenomena, i.e. reactants, charges transport and transfer and electrochemical reactions. First, the partial differential equations are solved in the steady state regime, then in the frequency domain in order to obtain the cell dynamic behaviour at different potentials. Experimental PEMFC impedance spectra are satisfactory reproduced over a relative large potentials range using only one set of model parameters. Numerical analysis of the key model parameters linked to the cell flooding state has been done. It is concluded that at least two impedance spectra at low and high potential are needed in order to discriminate the nature and the location of the cell degradations (anode or cathode, electrode or GDL). Based on a least square criterion, the model inversion is presented and several cell flooding scenarios have been precisely identified.     

Insulation diagnostic technique to assess the condition ofgenerator winding by measuring microwave associated with dielectricbreakdown

A partial discharge (PD) occurring in the insulation of the stator winding of the generator is a symptom of the deterioration of the insulation system, PD is a very short duration current and/or voltage pulse, about 1-5 ns. Generally a PD emits a broad band electromagnetic wave (~GHz). We developed a new system for detecting the microwave emitted from a PD. From the point of view of predicting an accident caused by the insulation breakdown of the generator winding, it is important to assess the condition of the winding. We applied our system to a dielectric breakdown test to know the relation between the characteristics of the microwave and the condition of the insulation system of the winding. The dielectric breakdown test is useful for estimating the minimum breakdown voltage which is a criterion of the insulation condition of the winding. Experimental results showed that this system was able to assess the insulation condition by measuring the number of the pulses of microwave frequency per voltage cycle     

Sensorless direct torque control for salient-pole PMSM based on extended Kalman filter fed by AC/DC/AC converter

In this paper, a new sensorless interior permanent magnet synchronous motor (IPMSM) drives method with extended Kalman filter (EKF) for speed, rotor position and load torque estimation is proposed. The direct torque control (DTC) technique for permanent magnet synchronous motor (PMSM) is receiving increasing attention due to the important advantages of the low dependence on motor parameters when compared with other motor control techniques. The Kalman filter is an observer for linear and non-linear systems and is based on the stochastic intromission, in others words, noise. The PMSM is fed by an indirect power electronic converter which is controlled by a sliding mode technique. The simulation tests performed for different operating conditions have confirmed the robustness of the overall system; and it is shown that the sliding mode technique has successfully minimized the different harmonics introduced by the line converter.     

Hydrogen detection by Schottky diodes

A compact hydrogen detector utilizing the Schottky barrier diode of a palladium-oxide-silicon structure as a sensor was developed. A bridge circuit compensated the temperature dependence of the diode characteristics. With the sensor operating at room temperature, the detector was able to respond to 2000 ppm H₂ in air within 10 s.     

Marangoni convection in a dielectric fluid layer with an AC electric field and nonuniform volumetric heat source due to incident radiation

The role of a uniform AC electric field and a nonuniform volumetric heat source arising due to an external incident radiation on the onset of Marangoni convection in a horizontal layer of an incompressible dielectric fluid is investigated. The bottom rigid surface is fixed at a constant temperature while the top free surface at which the surface tension acts is considered to be nondeformable and a Robin boundary condition on the perturbation temperature is invoked. The nonuniform internal heating within the fluid layer alters the conduction temperature profile from linear to nonlinear in the vertical coordinate. The linear stability of the quiescent basic solution is studied with respect to normal mode disturbances. The resulting stability eigenvalue problem with variable coefficient is solved numerically using the Galerkin method. The impact of governing parameters on the instability of the system is discussed thoroughly. The forces causing instability reinforce together and are found to be tightly coupled. It is observed that the strength of nonuniform heat source and the electric Rayleigh number is to hasten, while an increase in the Biot number is to delay the onset of Marangoni electroconvection. Finally, the results obtained under the limiting cases are shown to be in good agreement with those published earlier.     

Non-dimensional analysis of PEM fuel cell phenomena by means of AC impedance measurements

AC impedance or electrochemical impedance spectroscopy (EIS) is becoming a fundamental technique used by researchers and scientists in proton exchange membrane (PEM) fuel cell analysis and development. In this work, in situ impedance measurements are presented for a series of operating conditions in a 50 cm² fuel cell. The electrode charge transfer resistance was determined from the corresponding arcs of the Nyquist diagrams. The analyses were performed for H₂/O₂ and H₂/air operation at different stoichiometric factors and reactant gases humidification. Characteristic time scales of charge transfer processes at the different operating conditions were estimated from the corresponding Bode plots. These values were used for a non-dimensional analysis of the different fuel cell electrochemical and transport processes, namely electrochemical reaction versus GDL reactant transport. Fuel cell adapted Damkhöler numbers are thus presented, where the results indicate that the GDL diffusion transport is the limiting process for the cases under analysis, especially when air is used as oxidant. Additional analysis of channel convective mass transport versus GDL diffusive mass transport is also presented.     

区域交流电网故障引起直流系统换相失败的分析

以区域电网交流故障引发的一起直流系统逆变站极Ⅰ、极Ⅱ换相失败事故为例,从交流系统和直流系统的扰动分析人手,利用实时曲线图进行溯源,通过仿真计算验证和分析事故的主要原因,并结合事故特征及暴露问题,提出相应的改进建议,用以提升分析和处理换相失败等故障的能力,提高跨区电网直流输电系统的安全稳定运行水平。     

引入最佳冷化系数优化设计建筑楼宇冷热电系统

提出了“最佳冷化系数”的概念,对最佳冷化系数的应用及理论意义进行了论述,指出最佳冷化系数在建筑楼字冷热电（BCHP）设备选型中的意义,可使目前在空调制冷机组选型普遍偏大的问题得到改善,并结合实例给出了论证。     

Optimal composition of polymer electrolyte fuel cell electrodes determined by the AC impedance method

A proton exchange membrane fuel cell (PEMFC) electrode having a modified morphology of conventional Teflon (PTFE) bonded electrodes was studied using the AC impedance method. The electrode differs from other types of electrodes in the presence of a thin catalyst-supporting layer between the gas diffusion backing and the catalyst layer. The thickness and composition of the supporting layer were optimized on the basis of the information from AC impedance measurements. The optimal thickness of the supporting layer and its PTFE content turned out to be approximately 3.5 mg cm⁻² and 30 wt.%, respectively. The catalyst layer was cast on top of the supporting layer, from solution that has the proper ratio of ionomer Nafion and Pt/C catalyst. The optimal amount of the ionomer in the catalyst layer was approximately 0.8 mg cm⁻² when Pt loading was kept at 0.4 mg cm⁻². These values are rationalized in terms of the catalyst active area and the transport of the involved species for the electrode reaction.     

Fault diagnosis of photovoltaic modules through image processing and Canny edge detection on field thermographic measurements

Today, conventional condition monitoring of installed, operating photovoltaic (PV) modules is mainly based on electrical measurements and performance evaluation. However, such practices exhibit restricted fault-detection ability. This study proposes the use of standard thermal image processing and the Canny edge detection operator as diagnostic tools for module-related faults that lead to hot-spot heating effects. The intended techniques were applied on thermal images of defective PV modules, from several field infrared thermographic measurements conducted during this study. The whole approach provided promising results with the detection of hot-spot formations that were diagnosed to specific defective cells in each inspected module. These evolving hot spots lead to abnormally low performance of the PV modules, a fact that is also validated by the manufacturer's standard electrical tests.     

Experimental and analytical investigation of the cylindrical part of a metallic vessel reinforced by filament winding while submitted to internal pressure

In this work, we present an experimental and analytical investigation of a hydrogen storage vessel. This vessel is made of a carbon/epoxy envelope coated on a metal liner. In the theoretical part, an analytical model is proposed in which the laminate composite is assumed to be an anisotropic purely elastic material, whereas the liner is considered as an elasto-plastic material. The suggested analytical model provides an exact solution for stresses and strains on the cylindrical section of the vessel solution submitted to mechanical static loading. The aim of the experimental part is to validate the results of the theoretical model by manufacturing and testing some prototype vessels. Some analytical results are compared with the finite element solutions, a good correlation is observed.     

Water electrolysis enhanced by super gravity field for hydrogen production

Water electrolysis was carried out galvanostatically under a super gravity field. The relationships of cell voltage, ohmic voltage drop reduction and overpotential reduction with gravity coefficient (G) and current densities were studied. It was found that water electrolysis was noticeably enhanced by the super gravity field. Good linear relationships between cell voltage (U) and logarithm of G can be obtained at all current densities. Large cell voltage reduction was achieved under the super gravity field, especially under higher gravity and at higher current densities. Energy savings were about 9–17% at 0.5 A cm⁻² under the super gravity field. The cell voltage reduction was ascribed to ohmic voltage drop reduction and overpotential reduction. The share of ohmic voltage drop reduction (iΔR_G) in cell voltage reduction (ΔU_G) was higher than 70% at all current densities. At constant cell voltage, the rate of hydrogen produced under super gravity field increased noticeably compared to that under normal gravity condition.     

采用面向对象技术对格子Boltzmann方法模拟流场的编程实现

通过采用面向对象技术实现的格子Boltzmann对流场的模拟，展示了面向对象程序设计的优越性，充分说明了面向对象技术在数值模拟领域中的应用前景。     

Thermomechanics of damage and fatigue by a phase field model

In this paper, we present an isothermal model for describing damage and fatigue by the use of the Ginzburg–Landau (G–L) equation. Fatigue produces progressive damage, which is related with a variation of the internal structure of the material. The G–L equation studies the evolution of the order parameter, which describes the constitutive arrangement of the system and, in this framework, the evolution of damage. The thermodynamic coherence of the model is proved. In the last part of the work, we extend the results of the paper to a nonisothermal system, where fatigue contains thermal effects, which increase the damage of materials.     

Analysis of the voltage distribution in a motor stator winding subjected to steep-fronted surge voltages by means of a multiconductor lossy transmission line model

In this paper, the effect of steep-fronted voltage waveshapes infringing on a pulse-width-modulated (PWM) inverter fed induction motor is studied. The system, composed of a feeder cable and a stator winding, is modeled and simulated by using multiconductor transmission line theory in order to predict the voltage distribution among the coils of the stator winding. A recently developed time-domain equivalent circuit is used; it allows one to correctly describe the dielectric losses and the skin-effect in the conductors. The relationship among the voltage distribution inside the electrical insulation and parameters like the rise time of the applied voltage, the cable length, and the distributed losses is deeply discussed. Good agreement has been found among experimental and numerical results.